MikeJ said,

Note that even in Chicago "and" appears more than four times more often than "or". This is around the same ratio between the words that appears in random web text. So really, there's nothing to explain about Chicago, the anomaly is the other cities.

My guess is that real-estate-listing-domain text tends to contain a lot of "and"s and few "or"s because it tends to have a lot of lists of features of the property in question. And for whatever reason the real-estate culture in Chicago has evolved to include more narrative text and less dry feature-listing, while New York has evolved in the opposite direction. You could come up with a story about New Yorkers being slaves to efficiency while Chicagoans take the time to deal with one another on a human level, or something.

D.O. said,

Also, with ~700 words for average NYC announcements there is an average of 1.4 ors per announcement and for Chicago (~400 words) it's about 2 per. And why would you need to use or in a housing ad? Something like "e-mail or call"? Inquiring minds etc.

James said,

You wouldn't need to use 'or', but you know, "… ideal for use as nursery, guest bedroom, or study."

[(myl) Some examples:

Finished bedroom in basement with large unfinished space great for workshop or workout room.
Average approval within 58 days or less.
… bonus room on the main level can be used for a play rm or office …
Entertain with flair or relax by the pool.
For more information, contact XXX XXX at 678-XXX-XXX or XXX@EmailAddress
]

To James at 6:26am, and myl's reply: having lived in New York for 15 years, and looked at a number of apartments whose floor plan just shouldn't have existed, I can picture the realtor saying "this could be your x, OR you know, maybe your Y, OR then again…"